A wake-up receiver, in particular a low frequency, LF, wake-up receiver is used in the field of, for example, wireless sensors, access control, operator identification, real-time location systems, or active RFID tags. A wake-up receiver monitors the surrounding electromagnetic field in a certain frequency range and switches on a connected main system, for instance a micro-controller, a transceiver or else, every time that certain conditions are verified. A condition, for instance, is frequency and/or pattern detection. Thereby, the wake-up receiver generally works in background alongside the main system. Consequently, as long as conditions are not verified, the main system is switched off and only the wake-up receiver is operating. This mode of operation is called listening mode. Thus, the current consumption of a wake-up receiver is a very low, e.g. comparable to battery leakage current. A wake-up receiver is supposed to be always on. Since at low frequencies the received field is strongly influenced by the orientation of the transmitter or receiver, wake-up receivers usually have more than one channel in order to monitor the field in multiple directions. An additional antenna is mounted to a wake-up receiver for each receiving channel. This leads, for example, to a two or three dimensional field monitoring. As a consequence, the higher the number of channels, the higher the current consumption gets in listening mode.
In a state-of-the-art implementation of a wake-up receiver, the receiver operates in a so-called channel rotation mode: one channel after the other is cyclically checked for activity. When activity is detected, the currently active channel is enabled to receive data. The other channels having different antenna orientation are not used even in case they could deliver a better and stronger signal. The obvious deficiency is a shorter range of communication since the signal used to decode data is not necessarily the strongest signal available. In this way, the sensitivity of the receiver is reduced.
In another example of an existing wake-up receiver, the device has three channels offering the possibility to switch off one or more channels through register setting. With this device, channel rotation can be implemented only by using an external micro-controller which switches on and off a number of channels per time with a certain rotation time. The use of a micro-controller compromises the advantage in power saving of the wake-up receiver.
In a third implementation example, all available channels are simultaneously switched on during the check for activity resulting in high power consumption.